The detection of weak magnetic fields generated by individual electron and nuclear spins at the atomic level presents a significant challenge in the field of physics. Current quantum sensors, while achieving single-electron spin sensitivity, lack the atomic spatial resolution necessary for many applications. This research aims to address this limitation by developing a novel single-molecule quantum sensor. The ability to precisely measure these fields is crucial for advancing our understanding of fundamental physical phenomena and has far-reaching implications in various scientific disciplines. For instance, in materials science, this capability can help to characterize magnetic properties of materials at unprecedented resolutions, which is essential for designing and optimizing novel magnetic materials and devices. In biology, this allows scientists to study the magnetic properties of individual biomolecules, providing invaluable insights into biological processes. In the realm of quantum information science, the possibility of creating and manipulating atomic-scale magnetic fields paves the way for developing new types of quantum computing and sensing devices. Existing techniques, such as those employing nitrogen-vacancy (NV) centers in diamond, demonstrate excellent quantum-coherent properties but are limited in spatial resolution to tens of nanometers. This limitation arises from the physical separation of the sensor from the surface under investigation which is necessary for maintaining its quantum properties. In contrast, functionalizing probe tips with single molecules has improved the spatial resolution of scanning probe microscopes, enabling atomic-level imaging of chemical structures and surface potentials. However, these non-resonant techniques offer limited energy resolution, typically around 1.5 meV at 5 K, and lack the coherent control capabilities achievable through ESR. This study introduces a new approach that integrates the advantages of single-molecule manipulation with the high energy resolution and coherent control of ESR-STM, creating a mobile atomic-scale quantum sensor with unprecedented sensitivity and spatial resolution.

The research builds upon existing work in quantum sensing and scanning probe microscopy. Quantum sensors based on electron spin resonance (ESR), such as NV centers in diamond, have demonstrated excellent capabilities. However, their spatial resolution remains limited due to the sensor's physical distance from the surface of interest. Conversely, the use of single molecules functionalized to probe tips has enhanced the spatial resolution of scanning probe microscopy, particularly STM, allowing for atomic-scale imaging of various surface properties. However, the energy resolution and the ability to perform coherent control of the molecular probes have been significant limitations. Previous ESR-STM studies have demonstrated the sensing of magnetic moments of individual atoms using local ESR-active atoms as sensors. The limitation in these experiments, is the fixed nature of the sensing system, unlike the mobile system in the present study. All of these approaches used insulating substrates, which are often incompatible with many materials of interest. This research tackles these limitations by designing and constructing a fully mobile quantum sensor that is directly integrated onto an STM tip, eliminating the need for insulating layers. The researchers' strategy involves placing a planar molecule into an upright position on the tip apex, significantly reducing its interaction with the metal tip and making it suitable for ESR measurements.

The researchers fabricated their quantum sensor in situ using the atomic-scale manipulation capabilities of an STM at cryogenic temperatures and under ultra-high vacuum conditions. The sensor consists of a standing PTCDA molecule attached to an STM tip and augmented with additional Fe atoms. The PTCDA molecule serves as the sensing spin, while the Fe atoms provide spin polarization for driving and reading out the spin state. The fabrication involved several steps: First, the preparation of a clean Ag(111) surface and the deposition of individual Fe atoms and PTCDA molecules. Next, the manipulation of individual Fe atoms onto the STM tip apex using voltage pulses. Then the attachment of a single PTCDA molecule to the modified tip, subsequently lifting it into a standing configuration to minimize interaction with the metal tip. The process of assembling the quantum sensor was carefully controlled and monitored to achieve the desired configuration. The successful construction of the sensor was confirmed by STM imaging. The STM tip, modified to incorporate a PTCDA molecule in a standing configuration and equipped with Fe atoms, showed an ESR signature of a spin-1/2 system indicating the successful decoupling of the molecular spin from the metallic tip. The functionality of the integrated sensor as a quantum sensor was established by performing ESR measurements. In these experiments, a radio-frequency (RF) voltage was applied to the DC bias voltage, driving spin transitions in the PTCDA molecule. The change in the molecule's spin state was then measured through the tunneling magnetoresistance effect, which is a variation in the tunneling current depending on the orientation of the spin. The response of the sensor to external fields was characterized. This includes the determination of the sensor spin's quantization axis orientation, which is necessary for using it as a magnetic field sensor. The tip field, inherent to the sensor design, plays a significant role in these measurements and its orientation changes with the external magnetic field, causing a bistability in the response. The electric field sensitivity of the sensor was determined by varying the DC bias voltage while monitoring the resonance frequency. The electric field coupling strength was then calculated using the measured transduction parameter and the sensor-surface distance. Finally, the sensor was used to determine the electric and magnetic dipole moments of atomic objects (an Ag dimer and a single Fe atom) on the Ag(111) surface. This involved clearing a circular area around the target object and scanning the sensor across it while recording ESR spectra. The frequency shift during this process, after accounting for other factors, was used to extract the electric and magnetic dipole moments. A critical aspect of the methodology involved the disentangling of electric and magnetic dipole contributions, especially when determining the magnetic moment of the Fe atom where the uniaxial magnetic anisotropy plays a role. The spatial resolution of the quantum sensor was quantified by analyzing the frequency shift as a function of the lateral approach distance to the Fe atom, revealing sub-angstrom resolution.

The research successfully fabricated and characterized a mobile single-molecule quantum sensor capable of simultaneously measuring atomic-scale electric and magnetic fields. The sensor achieves an energy resolution of approximately 100 neV and a spatial resolution of better than 0.8 Å. The key findings include: 1. The successful fabrication and characterization of a novel single-molecule quantum sensor that integrates the high energy resolution and coherent control of ESR-STM with the high spatial resolution of single-molecule manipulation techniques. 2. The determination of the sensor's high energy resolution (~100 neV) and sub-angstrom spatial resolution. 3. Quantification of the sensor's sensitivity to both electric and magnetic fields with the determination of transduction parameters. 4. Successful demonstration of the sensor's ability to measure the electric and magnetic dipole moments of a single Fe atom and an Ag dimer on a Ag(111) surface. The experimental determination of the electric dipole moment of the Ag dimer (0.84 ± 0.02 D) agrees well with a value obtained using scanning quantum dot microscopy (SQDM) and highlights a higher sensitivity compared to SQDM. The magnetic dipole moment of the Fe atom (-3.2 ± 0.4 µB) was also determined and is consistent with previously reported values. The high sensitivity of the ESR-based detection scheme to electric fields was noted to be significantly higher than that of SQDM. This ability to simultaneously measure electric and magnetic fields at the atomic scale is a major advancement in the field. The study also highlights the significant improvement in energy resolution compared to conventional non-resonant STM techniques, owing to the coherent nature of the ESR process. The ability of the sensor to be used in conventional STM mode to image atomic structures is also crucial for precise positioning during sensing experiments.

The development of this mobile single-molecule quantum sensor addresses significant limitations of existing techniques in atomic-scale sensing. The achieved energy resolution (~100 neV) is a significant improvement over the energy resolution of standard STM techniques. This improvement stems from the narrowness of the detected ESR signal, which is determined by the spin relaxation and decoherence times of the sensor spin. The sub-angstrom spatial resolution surpasses existing methods and is a direct result of the sensor's close proximity to the surface and the exponential decay of the point spread function due to the screening effects in the STM junction. The ability to simultaneously measure electric and magnetic fields enables a more complete characterization of atomic-scale phenomena. The successful measurement of electric and magnetic dipole moments of single atoms showcases the potential of this sensor to investigate a wide range of materials and systems. The agreement between the measured electric dipole moment of the Ag dimer and the value obtained using SQDM validates the method and confirms the accuracy of the measurements. The observed electric field coupling strength is substantially larger than that of NV centers, offering an advantage for electric field sensing. The fact that the sensor can be used in a conventional STM mode for high resolution imaging, allows precise positioning on any surface suitable for STM imaging, before initiating sensing studies. The decoupling of the molecular spin from the metallic STM tip, through careful molecule-tip bonding, is a significant advance over previous ESR-STM techniques that required thin insulating layers.

This research has successfully demonstrated a fully integrated, mobile quantum sensor for atomic-scale electric and magnetic field measurements. The sensor combines high energy resolution (~100 neV) with sub-angstrom spatial resolution, surpassing the capabilities of previous methods. The ability to simultaneously measure electric and magnetic fields opens new avenues for investigating atomic-scale phenomena in materials science, biology, and quantum information science. Future research could focus on integrating pulsed ESR techniques to enhance the sensor's capabilities and expand the range of applications further. The potential for this technology to be used for biochemistry studies, particularly to investigate the electronic structures of catalytic centers in enzymes and other biomolecules on metal surfaces, is particularly promising.

While the study presents a significant advancement, several limitations warrant consideration. The success rate of fabricating functional quantum sensors was reported to be approximately 5%, suggesting improvements in the fabrication process could enhance yield and reliability. The electric and magnetic field coupling parameters are specific to the sensor design used. Further study into different configurations and molecules is necessary to determine the versatility and adaptability of this approach. The theoretical treatment of the electric field sensing assumed a simple parallel plate capacitor model for the STM junction, which could be refined to improve accuracy. The quantification of the spatial resolution relied on the analysis of peak widths and positioning precision, potentially overlooking other sources of uncertainty. Finally, the study has focused primarily on Fe atoms and Ag dimers as model systems. Further research is necessary to evaluate the sensor's performance with other molecules, materials, and operating conditions.